Grandpa's Viagra Addiction

Listen up, simpleton. The phrase "taking it harder" means the dad is emotionally distressed by his father's addiction. BUT, Viagra causes things to become physically hard. So the joke is a pun implying the dad is either severely struggling emotionally or, crudely, physically reacting the same way his dad is. You probably thought it was just about emotions because you're about as sharp as a marshmallow.